Petrified Forest National Park
Petrified Forest National Park is one of the world's largest and most colorful concentrations of petrified wood, mostly of the species Araucarioxylon arizonicum. Geography Petrified Forest National Park straddles the border between Apache County and Navajo County in northeastern Arizona. The park is about 30 miles (48 km) long from north to south, and its width varies from a maximum of about 12 miles (19 km) in the north to a minimum of about 1 mile (1.6 km) along a narrow corridor between the north and south, where the park widens again to about 4 to 5 miles (6 to 8 km).8 I-40, former U.S. Route 66, the BNSF Railway, and the Puerco River bisect the park generally east–west along a similar route. Adamana, a ghost town, is about 1 mile (1.6 km) west of the park along the BNSF tracks. Holbrook, about 26 miles (42 km) west of park headquarters along I-40, is the nearest city.89 Bisecting the park north–south is Park Road, which runs between I-40 near park headquarters on the north and U.S. Route 180 on the south. Historic Highway 180, an earlier alignment of the modern route, crosses the southern edge of the park. Like Route 66, it has deteriorated and is closed. Many unpaved maintenance roads, closed to the public, intersect Park Road at various points.10 The park covers about 146 square miles (380 km2).5 The Navajo Nation borders the park on the north and northeast. State-owned land, federal land controlled by the Bureau of Land Management, and private land, much of it used for cattle ranching, adjoin the other borders. The park’s elevation above sea level varies from a low of 5,340 feet (1,630 m) along the Puerco River to a high of 6,230 feet (1,900 m) at Pilot Rock; the average elevation is about 5,400 feet (1,600 m). The terrain varies from gentle hills and major petrified wood deposits in the south to eroded badlands in the north. Most of the park's intermittent streams—including Lithodendron Wash, Dead Wash, Ninemile Wash, and Dry Wash—empty into the Puerco River. In the southern part of the park, Cottonwood Wash and Jim Camp Wash flow into the Little Colorado River. Geology Petrified Forest National Park is known for its fossils, especially of fallen trees that lived in the Late Triassic period of the Mesozoic era, about 225 million years ago. During this period, the region that is now the park was near the equator on the southwestern edge of the supercontinent Pangaea, and its climate was humid and sub-tropical.11 What later became northeastern Arizona was a low plain flanked by mountains to the south and southeast and a sea to the west. Streams flowing across the plain from the highlands deposited inorganic sediment and organic matter, including trees as well as other plants and animals that had entered or fallen into the water. Although most organic matter decays rapidly or is eaten by other organisms, some is buried so quickly that it remains intact and may become fossilized.12 Within the park, the sediments containing the fossil logs for which the park is named are part of the Chinle Formation. The colorful Chinle, which appears on the surface in many parts of the southwestern United States and from which the Painted Desert gets its name, is up to 800 feet (240 m) thick in the park.12 It consists of a variety of sedimentary rocks including beds of soft, fine-grained mudstone, siltstone, and claystone—much of which is bentonite—as well as harder sandstone and conglomerate, and limestone.1112 Exposed to wind and water, the Chinle usually erodes differentially into badlands made up of cliffs, gullies, mesas, buttes, and rounded hills.12 Its bentonite clay, which swells when wet and shrinks while drying, causes surface movement and cracking that discourages plant growth. Lack of plant cover makes the Chinle especially susceptible to weathering.11 About 60 million years ago, tectonic movements of Earth’s crust began to uplift the Colorado Plateau, of which the Painted Desert is part.11 Eventually parts of the plateau rose to 10,000 feet (3,000 m) above sea level.11 This warping of Earth’s surface led to the gradual and continuing destruction of the plateau by erosion.13 An unconformity (break in the rock record) of about 200 million years occurs within the park, where erosion has removed all the rock layers above the Chinle except geologically recent ones. The Bidahochi Formation, laid down only 8 to 4 million years ago, rests directly atop the Chinle, and rocks laid down in the Jurassic, Cretaceous, and much of the Tertiary are absent. During the period of the Bidahochi deposition, a large lake basin covered much of northeastern Arizona. The older (lower) layers of the formation consist of fluvial and lacustrine (lake-related) deposits of silt, sand, and clay. The younger (upper) Bidahochi contains ash and lava from volcanoes that erupted nearby and as far away as southwestern Nevada.11 Although much of the Bidahochi has since eroded, a small part of it outcrops in the northern part of the park—on Pilot Rock in the park’s wilderness section and along the rim of the Painted Desert between Pintado and Tawa points.12 Exposed by erosion of the Bidahochi are volcanic landforms called maars (flat-bottomed, roughly circular volcanic craters of explosive origin). A maar vent can be seen from the Pintado Point lookout.11 During the Quaternary Period (1.8 million years ago up to today), deposits of windblown sand and alluvium covered much of the Chinle and Bidahochi. Older dunes range in age from 500,000 years at higher elevations in the northern part of the park to about 10,000 years in sandy drainage areas such as Lithodendron Wash. Stabilized by grasses and other vegetation, young dunes of about 1,000 years old are found throughout the park. Fossils During the Late Triassic, downed trees accumulating in river channels in what became the park were buried periodically by sediment containing volcanic ash. Groundwater dissolved silica (silicon dioxide) from the ash and carried it into the logs, where it formed quartz crystals that gradually replaced the organic matter. Traces of iron oxide and other substances combined with the silica to create varied colors in the petrified wood.14 In Petrified Forest National Park, most of the logs in the park retained their original external form during petrification but lost their internal structure. However, a small fraction of the logs and most of the park’s petrified animal bones have cells and other spaces that are mineral-filled but still retain much of their original organic structure. With these permineralized fossils, it is possible to study the cellular make-up of the original organisms with the aid of a microscope. Other organic matter—typically leaves, seeds, cones, pollen grains, spores, small stems, and fish, insect, and animal remains—have been preserved in the park as compression fossils, flattened by the weight of the sediments above until only a thin film remains in the rock.15 Much of the park’s petrified wood is from Araucarioxylon arizonicum trees, while some found in the northern part of the park is from Woodworthia arizonica and Schilderia adamanica trees. At least nine species of fossil trees from the park have been identified; all are extinct. The park has many other kinds of fossils besides trees. The Chinle, considered one of the richest Late Triassic fossil-plant deposits in the world, contains more than 200 fossil plant taxa. Plant groups represented in the park include lycopods, ferns, cycads, conifers, ginkgoes, as well as unclassified forms.14 The park has also produced many fossil vertebrates—including giant crocodile-like reptiles called phytosaurs, large salamander-like amphibians called Buettneria, and early dinosaurs—and invertebrates, including freshwater snails and clams. Climate See also *Petrified Forest Category:Arizona Category:Triassic paleontological sites